Increasing environmental restrictions and regulations are causing diesel engine manufacturers and packagers to develop technologies that improve and reduce the impact that operation of such engines have on the environment. As a result, much design work has gone into the controls that operate the combustion process within the engine itself in an attempt to increase fuel economy and reduce emissions such as NOx and particulates. However, given the operating variables and parameters over which a diesel engine operates and given the tradeoff between NOx and particulate generation, many engine manufacturers and packagers have found it useful or necessary to apply exhaust after-treatment devices to their systems. These devices are used to filter the exhaust gas flow from the diesel engine to remove or reduce to acceptable levels certain emissions. Such devices are particularly useful in removing exhaust particulates, or soot, from the exhaust gas flow before such soot is released into the environment.
One such exhaust after-treatment device is called a Diesel Particulate Filter (DPF). The DPF is positioned in the exhaust system such that all exhaust gases from the diesel engine flow through it. The DPF is configured so that the soot particles in the exhaust gas are deposited in the filter substrate of the DPF. In this way, the soot particulates are filtered out of the exhaust gas so that the engine or engine system can meet or exceed the environmental regulations that apply thereto.
While such devices provide a significant environmental benefit, as with any filter, problems may occur as the DPF continues to accumulate these particulates. After a period of time, the DPF becomes sufficiently loaded with soot such that the exhaust gases experience a significant pressure drop passing through the increasingly restrictive filter. As a result of operating with an overly restrictive filter, the engine thermal efficiency declines because the engine must work harder and harder simply to pump the exhaust gases through the loaded DPF. Besides the reduced thermal efficiency, a second and potentially more dangerous problem may occur. Because the soot accumulated in the DPF is flammable, continued operation with a loaded DPF raises the serious potential for uncontrolled exhaust fires if, and when, the accumulated soot is eventually ignited and burns uncontrollably.
To avoid either occurrence, one of several possible filter heating devices is typically incorporated upstream of the DPF to periodically clean the filter. These filter heating devices are used periodically to artificially raise the temperature of the exhaust stream to a point at which the accumulated soot will self-ignite. When initiated at a time before the soot loading of the DPF becomes excessive, the ignition and burn-off will occur in a safe and controlled fashion. This process of burning the soot in such a controlled manner is called regeneration. The control of the method to generate the supplemental heat necessary to increase the temperature in the DPF is critical to safe and reliable regeneration. Typically, the acceptable temperature range for regeneration is from 600 to 900° C. Temperatures below this range are insufficient to ignite the accumulated soot, and temperatures above this range may cause thermal damage to the filter media.
Many methods have been devised to provide the auxiliary heat necessary to initiate regeneration. For example, the operating parameters of the diesel engine may be modified in such a manner to cause the exhaust temperature to rise to a level sufficient for proper operation of the downstream particulate filter. It is also possible to inject hydrocarbon fuel into the exhaust of a diesel engine immediately before the exhaust passes through a diesel oxidation catalyst (DOC) positioned upstream of the particulate filter. The DOC converts the excess hydrocarbon fuel into heat by means of the catalytic reaction of the catalyst, thus increasing the exhaust gas temperature prior to its passage through the particulate filter. Supplemental heat may also be generated in the exhaust flow by use of an auxiliary electrical heater, or a microwave heater, placed within the exhaust path. This supplemental heat is added to the exhaust gas prior to its passage through the particulate filter. As an alternative to the use of a microwave or electric heater, another method of filter regeneration uses a fuel-fired burner to heat the exhaust gas prior to the DPF. Such a burner requires a diesel fuel supply, an auxiliary air supply, and an ignition system.
The rate at which soot accumulates in the filter depends primarily upon the operating regime of the engine. As such, besides the selection of the particular method or device to be used to heat the exhaust gas to enable regeneration, the engine manufacturer or packager must also determine when to initiate the regeneration process. If regeneration is initiated too soon, when the DPF is only lightly loaded, the process will be inefficient. If the regeneration is not initiated until the DPF is heavily loaded, the overall engine efficiency would have been unduly reduced as discussed above, and there is a risk that the soot may self-ignite and/or that the burn may be unsafe and uncontrolled.
In an attempt to properly determine when to initiate the regeneration process, several sensors and control algorithms have been developed. These sensors and control algorithms may be used to estimate the soot loading of the DPF so that regeneration can be initiated only after soot loading could cause an engine efficiency reduction but before excessive loading occurs that would actually result in such an efficiency reduction and increased potential for self-ignition. However, these control algorithms typically depend on the monitoring of certain triggering events which generate boolean (i.e. true/false) responses based on comparisons to some predetermined limit. These boolean responses form the inputs for the control algorithm.
One drawback of this method is that it may have a very discrete response. That is, one triggering event may determine when regeneration is initiated without taking a variety of other relevant factors into account. Also, by basing the initiation of regeneration on the boolean response of certain triggering events, there is the potential that a faulty or broken sensor could, for example, initiate regeneration events much more frequently than needed, or could delay regeneration far longer than is desirable. A control system that relies on analog responses, and that takes into account the values of a variety of relevant parameters could avoid this problem
It would therefore be desirable to have a system and method of controlling the initiation of the regeneration process, wherein the system and method are capable of evaluating the analog responses of a variety of factors such that regeneration events are efficiently controlled and initiated when truly needed. Embodiments of the invention provide such a system and method of controlling initiation of the regeneration process. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.